48.72% of all pets were POOR QUALITY (gray)
23.42% of all pets were COMMON QUALITY (white)
23.59% of all pets were UNCOMMON QUALITY (green)
4.27% of all pets were RARE QUALITY (blue)
It appears blizzard has set the numbers at 48/24/24/4% for poor, common, uncommon, and rare, respectively. However, I won't know with 99% confidence +/- 1% for another ~16,000 or so wild pets battled

Assuming 48/24/24/4% are the correct numbers, here are a few statistics for the pet collector:
- 1 in 25 of all wild pets are rare
- If you are looking for a specific rare pet, you will encounter 17 lower quality versions of that pet before it is more than likely (>50%) that you will have found the rare version. Note: this is not a guarantee. "More than likely" means that, statistically, just over half of the people searching for a specific rare pet will find it after they encounter 17 lower quality versions of that pet. Some will find it in far fewer encounters, some far more.
- 12 in 25 of all wild pets are either common or uncommon
- 25% of people searching for a specific wild rare will have to endure finding 34 lower quality versions of the pet they're after before a rare appears, and 10% will find 57 lower quality versions
- If you intend on collecting all 259 wild pets in rare form only, you're looking at encountering 6,475 different wild pets on average to catch them all
Now, for the nuts and bolts. If you're not interested in specifics, feel free to tune out at this point.
Data was collected from wild pets in WotLK and Cataclysm zones only. Every encounter was against a group of 3 wild pets. It is assumed that no pet has a higher likelihood of being found in rare form than any other, and no zone produces more rare pets than any other. Some pets, mostly pets with lengthy or specific spawn timers, occur only once per battle, such as the Scourged Whelpling. That means it is less likely to find one rare in the wild than, say, a Rabbit, because the rabbit can occur 2 or 3 times in the same battle. Updates will be provided as I collect more information. Suggestions/corrections/complaints are welcome.